1. Field of the Invention
This invention relates generally to a device for controlling a gas flow to a fuel cell and, more particularly, to a device for controlling the flow of air to the cathode side of a fuel cell, where the device includes a member for blocking a predetermined number of cathode side flow channels during low load conditions.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid-polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of each MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of each MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells from one cell to the next cell as well as out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
FIG. 1 is a cross-sectional view of a fuel cell 10 of the type discussed above. The fuel cell 10 includes a cathode side 12 and an anode side 14 separated by an electrolyte membrane 16. A cathode side diffusion media layer 20 is provided at the cathode side 12, and a cathode side catalyst layer 22 is provided between the membrane 16 and the diffusion media layer 20. Likewise, an anode side diffusion media layer 24 is provided at the anode side 14, and an anode catalyst layer 26 is provided between the membrane 16 and the diffusion media layer 24. The catalyst layers 22 and 26 and the membrane 16 define an MEA. The diffusion media layers 20 and 24 are porous layers that provide for input gas transport to and water transport from the MEA. Various techniques are known in the art for depositing the catalyst layers 22 and 26 on the diffusion media layers 20 and 24, respectively, or on the membrane 16.
A cathode side flow field plate or bipolar plate 18 is provided on the cathode side 12 and an anode side flow field plate or bipolar plate 30 is provided on the anode side 14. The bipolar plates 18 and 30 are positioned between the fuel cells in a fuel cell stack. A hydrogen gas flow 28 from flow channels (not shown in FIG. 1) in the bipolar plate 30 reacts with the catalyst layer 26 to disassociate the hydrogen ions and the electrons. Airflow 36 from flow channels (not shown in FIG. 1) in the bipolar plate 18 reacts with the catalyst layer 22. The hydrogen ions are able to propagate through the membrane 16 where they electro-chemically react with the airflow 36 and the return electrons in the catalyst layer 22 to generate water.
As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. During operation of the fuel cell, water from the MEAs and external humidification may enter the anode and cathode flow channels. The water may accumulate in the flow channels, especially at low loads where the flow rate of the reactant gas will be low. It has been shown that water accumulation becomes a problem at current densities below 0.2-0.4 A/cm2 and a cell stoichiometry of 2-4.
A flow channel in which liquid water has accumulated will have a lower reactant flow than the flow channels where no water has accumulated. Because the flow channels are in parallel, the input gas may not flow through a channel with water accumulation, thus preventing the water from being forced out and allowing for increased water accumulation therein. Those areas of the membrane that do not receive input gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. Significant water accumulation in a single cell could result in severe reactant blockage to that cell and cause the cell to fail. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.
It is possible to purge the accumulated water in the flow channels by forcing the anode gas or the cathode gas through the flow channels at a higher flow rate than is necessary to meet the output load demands (higher stoichiometry). However, there are many reasons not to use the hydrogen fuel as a purge gas, including reduced economy, reduced system efficiency and increased-system complexity for treating elevated concentrations of hydrogen in the exhaust gas stream. For these reasons, it will be desirable to at least minimize the water accumulating in the anode side flow channels of the fuel cells.
Reducing accumulated water in the channels can also be accomplished by reducing inlet humidification. However, it is desirable to provide some relative humidity in the anode and cathode gases so that the membrane in the fuel cells remains hydrated. A dry inlet gas has a drying effect on the membrane that could increase the ionic resistance, and limit the membrane's long-term durability.
Accumulated water in the cells can also reduce performance of the fuel cell when operated in an environment where the temperature goes below 0° C. The accumulated water could also lead to mechanical damage in these environments.